function isEmpty1(obj){
    return JSON.stringify(obj) === '{}';
}
function isEmpty2(obj){
    return Object.keys(obj).length === 0;
}
function isEmpty3(obj){
    return Object.getOwnPropertyNames(obj).length === 0;
}
function isEmpty4(obj){
    let flag = true;
    for(key in obj){
        if(key){
            flag = false;
            break;
        }
    }
    return flag;
}
function isEmpty5(obj){
    return Reflect.ownKeys(obj).length === 0;
}
let key = Symbol('a')
let o = {[key]:1}
console.log('1:',isEmpty1(o))
console.log('2:',isEmpty2(o))
console.log('3:',isEmpty3(o))
console.log('4:',isEmpty4(o))
console.log('5:',isEmpty5(o))
